Tenth Doctor Month: Quote Of The Day #16

Martha: But are we safe? I mean, can we move around and stuff?
The Doctor: Of course we can. Why do you ask?
Martha: It's like in the films. You step on a butterfly, you change the future of the human race.
The Doctor: Tell you what then, don't step on any butterflies. What have butterflies ever done to you? 

The Shakespeare Code (2007)